An electrocardiogram (ECG) is well known in the art as a transthoracic interpretation of electrical activity of the heart as detected by electrodes. Electrical impulses in the heart originate in the sinoatrial node and travel through an intrinsic conducting system to the heart muscle. The impulses stimulate myocardial muscle fibers to contract and thus induce systole. These electrical impulses may be detected by a set of three electrodes (electrical contacts) selectively placed on the skin. An ECG represents a voltage measured between pairs of these electrodes and the muscle activity that the electrodes detect from different directions, or vectors. A typical shape of the systolic portion of an ECG output signal, plotted as a time-varying voltage, is known to those skilled in the art as the “QRS complex” in which Q, R, and S designate typical feature shapes that correspond to events in a cardiac cycle.
Present day implantable cardiac devices such as pacemakers, implantable defibrillators, and the like, include a monitoring function that may use either intracardiac or subcutaneous electrodes to sense electrical signals from the heart, in a similar fashion to a conventional, external ECG. Intracardiac electrodes are implanted directly into the heart tissue; whereas subcutaneous electrodes are fixedly attached to the housing of the implantable device. If subcutaneous electrodes are used, the resultant recording is called a subcutaneous electrocardiogram (SECG). Existing subcutaneous monitoring devices typically use only two electrodes (one pair), thus providing only one recording channel. Because the separation between the electrodes is small (due to the device size), the distance to the heart is relatively large, and skeletal muscle is in close proximity to the device, signals detected by subcutaneous electrodes are more susceptible to noise than are signals detected by implanted leads placed directly on the heart tissue. Signals detected by subcutaneous electrodes are also highly dependent upon orientation of the monitoring device with respect to the heart. Proper placement of the monitoring device is required during implantation to ensure optimal signal amplitude. The implanted device is preferably positioned so as to maximize the QRS signal amplitudes detected.
After implantation, it is generally difficult to identify movement of the device away from its optimal position. Such a displacement may cause a decrease in electrocardiographic signal amplitudes, resulting in a poor quality SECG. If such a decrease in signal strength occurs, existing devices generally cannot distinguish whether or not the reason is due to a shift in the device position. To confirm the orientation of the device, inspection by a physician currently requires the patient to travel to the physician and possibly to receive an X-ray. For at least these reasons, an improved method of determining the orientation of implantable heart monitoring devices is needed.